Regulatory requirements mandate the implementation of a leak detection system for automotive evaporative emissions (EVAP) control systems, in which the detection system must light a malfunction indicator when a leak of sufficient size is detected. Should a fuel filler pipe cap not be replaced after refueling, the leak detection system would detect the missing cap as a leak in the EVAP system and light the indicator. Such a common oversight as failure to replace the filler cap, however, is not in actuality a leak in the EVAP system intended to be monitored by the system and essentially amounts to a false leak indication. Because an excessive number of EVAP repairs may trigger a recall for a given make of automobile, it is desirable to distinguish operator error from actual leaks. As such, the regulatory agencies in question allow the separate detection and indication of a missing fuel filler pipe cap in order to prevent unneeded service to the EVAP system.
Various mechanically actuated filler cap detection switches have been devised, including U.S. Pat. Nos. 4,583,072 (Matsushita), 2,019,393 (Carah), and 3,572,305 (Moragne). A drawback of such devices is that contamination can collect on their moving parts causing them to stick and malfunction. Other mechanically operated devices for preventing unauthorized access to the fuel tank disclosed in U.S. Pat. Nos. 3,878,507 (Medlock) and 3,792,435 (Pace, et. al.). These also have the disadvantages discussed above.